XoxF and the Calvin-Benson Cycle Mediate Lanthanide-Dependent Growth on Methanol in Bradyrhizobium and Sinorhizobium

Nodule-forming bacteria play crucial roles in plant health and nutrition by providing fixed nitrogen to leguminous plants. Despite the importance of this relationship, how nodule-forming bacteria are affected by plant exudates and soil minerals is not fully characterized. Here, the effects of plant-derived methanol and lanthanide metals on the growth of nitrogen-fixing Rhizobiales are examined. Prior work has demonstrated that select Bradyrhizobium strains can assimilate methanol only in the presence of lanthanide metals; however, the pathway enabling assimilation remains unknown. In this study, we characterize Bradyrhizobium diazoefficiens USDA 110, Bradyrhizobium sp. USDA 3456, and Sinorhizobium meliloti 2011 to determine the pathways involved in methanol metabolism in previously characterized strains, other clades of Bradyrhizobium, and the more distantly related Sinorhizobium. Based on genomic analyses, we hypothesized that methanol assimilation in these organisms occurs via the lanthanide-dependent methanol dehydrogenase XoxF, followed by oxidation of formaldehyde via the glutathione-linked oxidation pathway, subsequent oxidation of formate via formate dehydrogenases, and finally assimilation of CO2 via the Calvin-Benson-Bassham (CBB) cycle. Transcriptomics revealed upregulation of the aforementioned pathways in Bradyrhizobium sp. USDA 3456 during growth on methanol. Enzymatic assays demonstrated increased activity of the glutathione-linked oxidation pathway and formate dehydrogenases in all strains during growth on methanol compared to succinate. 13C-labeling studies confirmed the presence of CBB intermediates and label incorporation during growth on methanol. Our findings provide multiple lines of evidence supporting the proposed XoxF-CBB pathway and, combined with genomic analyses, suggest that this metabolism is widespread among Bradyrhizobium and Sinorhizobium species. ImportanceNitrogen-fixing soil bacteria such as Bradyrhizobium and Sinorhizobium promote plant growth while reducing dependence on artificial, energy-intensive fertilizers. Numerous studies have attempted to increase bacterial nitrogen fixation and colonization of plant tissues by identifying the micronutrients and plant exudates that promote successful symbiotic relationships. Among the compounds encountered by rhizobacteria, lanthanides have received little attention, despite reports that plant growth is affected by the presence of lanthanides. We characterized three agriculturally relevant Bradyrhizobium and Sinorhizobium strains, demonstrated that they gain the capacity to utilize methanol when lanthanides are present, and experimentally determined the pathway by which this metabolism occurs. This study provides a foundation for understanding the impacts of bacteria-mediated lanthanide metabolism on plant growth.